If you want reliable tool then I would suggest to use Migrate Mac OLM to Gmail Converter. This software provides option to import OLM files into Gmail account in a simple manner. Users can batch convert OLM to Gmail file to select single and multiple files using the batch feature. You can also convert the damaged or normal file and save data in various file formats like PST, MSG, EML, PDF, HTML.https://www.kdetools.com/mac/olm/gmail/